What you need

Steps

Open the aircraft and tap Measure wing with camera, or tap Measure with camera in the CG calculator.
Lay the wing flat, underside up, on the plain background.
Lay the paper flat on the wing, fully inside its outline.
Tap Take photo. Hold the iPhone level, straight above the middle of the wing, and take the picture.
Check that each handle sits exactly on a corner of the paper, then tap Continue.
For a half wing, tap the root edge. For a whole wing, drag the two handles onto the centerline.
Drag any orange or blue corner that isn't on the wing's edge, then tap Show results.
Check the sizes against a ruler, then tap Save.

Half wing is the default. One half fills more of the photo, so it measures more accurately.
A credit card works, but it's small, so tiny errors grow across the wing. Use paper if you can.
If the paper can't go on the wing, choose Beside the wing and enter the wing's thickness.
Orange handles mark the leading edge, and blue handles mark the trailing edge.

The paper's corners weren't found: Use even light and a background darker than the paper. Tap Find again, or drag the four handles onto the corners.
The outline is wrong: Tap Place corners by hand, then drag each corner onto the wing's edge.
The wing measures too big: Check that the paper lies on the wing itself, not on the table beside it.

CG: Center of gravity: the point where the model balances. Too far back makes it unstable. Too far forward makes the nose hard to lift.
MAC: Mean aerodynamic chord: the average front-to-back width of a tapered wing. CG positions are given as a percentage of it.
Leading edge: The front edge of a wing, stab or control surface.
Trailing edge: The back edge of a wing, stab or control surface.
Root: The inner end of a wing or stab, where it meets the fuselage.
Chord: The width of a wing or control surface, from its leading edge to its trailing edge.